A magnetorheological fluid damper which changes a damping force by causing a magnetizing field to act on a flow passage in which magnetorheological fluid passes and changing an apparent viscosity of the magnetorheological fluid is known as a damper installed in a vehicle such as an automotive vehicle.
JP2009-228861A discloses a magnetorheological fluid damper which changes a damping force by an electromagnetic force of an electromagnetic coil.
This magnetorheological fluid damper includes first and second fluid chambers partitioned by a piston, a main flow passage (piston though passage) which is open on both ends of the piston and allows communication between the first and second fluid chambers, and a bypass flow passage which is open on the both ends of the piston and parallel with the main flow passage and allows communication between the first and second fluid chambers.
The main flow passage is so configured that a magnetizing field generated by an electromagnetic coil acts on magnetorheological fluid passing therein. By adjusting a current value of the electromagnetic coil, flow resistance of the magnetorheological fluid passing in the main flow passage is adjusted and a damping force generated by the magnetorheological fluid damper changes.
The bypass flow passage is so configured that the magnetorheological fluid passing therein is unlikely to be affected by the magnetizing field generated by the electromagnetic coil. A pressure fluctuation caused when the current value of the electromagnetic coil is adjusted is alleviated by flow resistance of the magnetorheological fluid passing in the bypass flow passage.